1. Field of the Invention
The present invention relates to a zinc-phosphate glass composition suitable for continuously making glass fibers which may be used as a reinforcement for high temperature polymer composites.
2. Background of the Invention
The reinforced plastic industry has been using glass fibers in various forms for reinforcing polymer matrices; the glass phase provides increased stiffness and strength to the polymer phase. However, up until now most, if not all, commercially available glass fibers produced for reinforcement were E-glass fibers, i.e. calcium-alumino-silicate glass fibers. However, one disadvantage of these fibers is that the processing temperatures used in making these glass/polymer composites are well below the softening point of these silicate fibers. Therefore, because these fibers are brittle, they are quite prone to breakage during compounding. As a result, the fibers within the composite tend to get chopped up during compounding such that these shortened silicate glass fiber-reinforced polymer composites possess less strength than if the fibers would not have been shortened. Another shortcoming is that these composites are limited as to the proportion of fibers which can be added to the polymer; anything greater than about 20% by volume causes the viscosity of the mixture to be to high for forming at the processing temperature.
With the invention of base alkali metal zinc phosphate glasses exhibiting transition temperatures below 450.degree. C. which possessed good resistance to attack by water in U.S. Pat. No. 4,940,677 (Beall et al.), the possibility of forming composites where a zinc-phosphate glass and a polymer were co-deformable at the polymer processing temperature became possible. That patent discloses glass compositions consisting essentially, expressed in terms of mole percent on the oxide basis, of about 10-35% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions selected from the group 0-25% Li.sub.2 O, 0-25% Na.sub.2 O, and 0-25% K.sub.2 O, 23-55% XnO, 28-40% P.sub.2 O.sub.5, and up to 35% total of the following optional ingredients from the group 0-6% Al.sub.2 O.sub.3, 0-8% B.sub.2 O.sub.3, 0-8% Al.sub.2 O.sub.3 +B.sub.2 O.sub.3, 0-15% CuO, 0-5% F, 0-35% PbO, 0-35% SnO, 0-35% PbO+SnO, 0-5% ZrO.sub.2, 0-4% SiO.sub.2, and 0-15% MgO+CaO+SrO+BaO+MnO, consisting of 0-10% MgO, 0-10% CaO, 0-10% SrO, 0-12% BaO, and 0-10% MnO.
However, although the above compositions were capable of being drawn into fiber, the ability to continuously draw fibers was not possible. Not only did the produced fiber contain nodules, but additionally the glass devitrified during fiber drawing. Both of these shortcomings of the process were a result of the glass wetting the platinum bushings which resulted in the glass backing up the bushing via capillary wetting and further causing a build-up of the glass upstream from the orifice. Because of this "wetting" problem, the fiber drawing process caused regular breakage of the fiber, as well as shutdowns in order to unclog the orifice by cleaning out the devitrified glass.